1. Field of the Invention
The present invention relates to an improvement of circuit construction of an all pass filter of second order used for equalization of necessary transmission characteristic in waveform transmission of a digital signal and the like.
2. Description of the Related Art
For equalization of a digital signal transmission waveform, it is necessary to equalize both characteristics of amplitude characteristics and group delay characteristics (or phase characteristics) but since equalization of the group delay characteristics is done after equalization of the amplitude characteristics, equalization by an all pass filter having flat amplitude characteristic is required. Also, as an equalizer for this purpose, there are an automatic equalizer and a fixed equalizer and here the fixed equalizer is adopted.
In a case that a group characteristics of comparatively low frequency band is equalized by a fixed equalizer, since various all pass filters using an operational amplifier (op-amp) are developed, it is equalized easily by using them. In these equalizers, since there is one which is able to change a center frequency of equalization characteristics (a center frequency of phase characteristics) and its Q independently with each other (or at least one of them independently), circuit adjustment is easy if they are used and in addition if that circuit is constituted by an active filter, it has a merit in circuit construction that it can be realized without an inductor.
However, in a case that group delay characteristics of such high frequency band that an op-amp can be used therein is equalized, an all pass circuit of passive type based on conventional circuit network theory or an all pass circuit is constituted by a transistor circuit.
In a case that the all pass circuit of passive type is utilized, since there is no such freedom that a center frequency or Q (or one of them) can be changed, which is possible in an all pass filter having an op-amp, if anyone of them is intended to change, all element values constituting a circuit must be changed and thus circuit adjustment becomes complicated and very inconvenient. Also, a suitable method for constituting an all pass filter by using a transistor circuit is not developed.
The object of the present invention is to provide an all pass filter capable of changing a center frequency thereof and Q independently with each other or anyone of them independently in such high frequency band that an op-amp can not be utilized.
In order to achieve the above object, the present invention proposes all pass filters having the following constructions.
(1) An all pass filter comprising a tuning circuit of series resonance type containing a Q adjusting resistor, a gain control circuit for an input signal applied to said tuning circuit, and a subtraction circuit for subtracting the input signal from an output signal of said tuning circuit.
(2) An all pass filter described in item (1) wherein said gain control circuit includes a resistor dividing circuit and a first amplifier circuit for in phase amplifying an output of said resistor dividing circuit, and said subtraction circuit includes a second amplifier circuit for inversely amplifying the input signal, a third amplifier circuit for in phase amplifying the output signal of said tuning circuit and a common load for the second and the third amplifier circuits.
(3) An all pass filter described in item (1) wherein said gain control circuit includes a resistor dividing circuit and a first amplifier circuit for in phase amplifying an output signal of said resistor dividing circuit, and said subtraction circuit a second amplifier circuit for inversely amplifying the input signal, a third amplifier circuit for in phase amplifying an output signal said tuning circuit and a resistor synthesizer circuit for synthesizing the output signals of said second and third amplifier circuits.
(4) An all pass filter comprising a tuning circuit of M connection parallel resonance type containing a Q adjusting resistor, a gain control circuit for an input signal applied to said tuning circuit, and an adder circuit for adding the input signal to an output signal of said tuning circuit.
(5) An all pass filter described in item (4) wherein said gain control circuit includes a resistor dividing circuit connected to a M connection coil of the tuning circuit, and the adder circuit a fourth amplifier circuit for in phase amplifying the, input signal, a fifth amplifier circuit for in phase amplifying an output signal of said tuning circuit and a common load for said fourth and fifth amplifier circuits.
(6) An all pass filter described in item (4) wherein said adder circuit includes a fourth amplifier circuit for in phase amplifying the input signal and a fifth amplifier circuit for in phase amplifying an output signal of said tuning circuit, and said gain control circuit includes a resistor synthesizer circuit connected to said fourth and fifth amplifier circuit.
(7) An all pass filter comprising a tuning circuit of M connection parallel resonance type containing a Q adjusting resistor, a gain control circuit for an input signal applied to said tuning circuit, and a subtraction circuit for subtracting an output signal of said tuning circuit from the input signal.
(8) An all pass filter described in item (7) wherein said gain control circuit includes a resistor dividing circuit, and said subtraction circuit includes a sixth amplifier circuit for inversely amplifying an output signal of said tuning circuit, a seventh amplifier circuit for in phase amplifying an output signal of said tuning circuit and a load in which an output difference current between said sixth amplifier circuit and said seventh amplifier circuits flows.
(9) An all pass filter comprising a tuning circuit of tap input and parallel resonance type containing a Q adjusting resistor, a gain control circuit for an input signal applied to said tuning circuit, and a subtraction circuit for subtracting an output signal of said tuning circuit from the input signal.
(10) An all pass filter described in item (9) wherein said gain control circuit includes a resistor dividing circuit, and said subtraction circuit includes a sixth amplifier circuit for inversely amplifying the input signal, a seventh amplifier circuit for in phase amplifying an output signal of said tuning circuit and a load in which an output difference current between said sixth amplifier circuit and said seventh amplifier circuit.
(11) An all pass filter comprising a tuning circuit of phase inversion and parallel resonance type containing a Q adjusting resistor, and a gain control circuit for a difference signal obtained by applying the input signal to an output signal of said tuning circuit.
(12) An all pass filter described in item (11) wherein said gain control circuit includes a resistor dividing circuit and an impedance converter is connected to an output side of said tuning circuit.
(13) An all pass filter comprising a tuning circuit of parallel resonance type containing a Q adjusting circuit, a single winding transformer having a tap terminal connected to said tuning circuit, and a gain control circuit for a difference signal obtained by applying an input signal to an output signal of said tuning circuit.
(14) An all pass filter according to item (13) wherein said gain control circuit includes a resistor synthesizer circuit and an impedance converter is connected to an output side of said tuning circuit.